AJAX alone is worth very little without the server-side languages, it's simply a way to grab data off the server / db without doing a page reload. Used the right way its a great (and very cool) tool, but it's not going to stand on its own, its essentially Javascript back in its glory days. Gmail, Meebo, Digg, Basecamp, etc., all the Web 2.0 companies implement AJAX in various ways but well they wouldn't exist without PHP, RoR, Perl, cgi etc.
Now PHP has a huge following, it has had this for a long time now and its definitely up there as one of the more dominant languages for web development. It's easy to use, (very easy for beginners to pick up) and deployment is cheap (about 95% of hosts probably support it). I'd say PHP will continue to be in forefront of web development for the coming year. RoR (Ruby on Rails) has made a lot of noise over the past year, and its a great language / framework to base your web apps on, but most hosts don't support it ATM (Dreamhost's a good host if you wanna go the RoR way). Django (based on Python) interests me a lot as well but it's in the same position as Ruby is (poor hosting support). And well if you like 'em IIS / Windows Servers there's always ASP.net. You can't really go wrong with any of the languages mentioned, (and there's a lot more, I'm sure there are Coldfusion fans out here as well) just weigh in the ins and outs of each and stick with it.
Oh, and yep, as Baddudes says, at the end of the day, all these languages will be spitting out html.
